U.S. Pat. No. 4,121,874 discloses a power and manual hydraulic brake system for an airplane, along with anti-skid control for the same. A relay valve and a shuttle valve coact with the pilot's manual master-cylinder to effect control of the power-assisted mode brake cylinder.
However, there is no isolation valve, as there is in the present invention. When the system of the patent is in the manual mode the passageway that connects from the manual master-cylinder to the power brake cylinder remains a part of the hydraulic system. Thus, actuation of the pilot's master-cylinder displaces fluid through that passageway and into the power brake cylinder to operate it, even though the power system is inoperative and no further functioning in that system takes place.
This displacement of fluid reduces the maximum hydraulic pressure obtainable during manual operation by about 25% to 33% in most installations of what it would be if the same system were wholly manual.
This is a very serious reduction of performance capability in the manual mode.